1. Field of the Invention
The present invention generally relates to resins and more particularly to an improved polyester resin for coil coatings and a method of making the same.
2. Prior Art
Modern liquid coatings for application to metallic coil substrates conventionally include linear polyester resin which has been cross-linked with melamine or a similar resin and imparts to the cured film provided by the coating the desired hardness and solvent resistance. Such polyester resins are preferably obtained by reacting di-functional acids and alcohols together. Terephthalic acid has been the aromatic di-functional acid of choice in preparing such polyester resins because it lends to the ultimate cured film improved hardness and greater damage resistance than other aromatic di-functional acids at equivalent levels of flexibility. Unfortunately, terephthalic acid has two major disadvantages. In this regard, it tends to impair the solubility of the polyester resin in common organic solvents. It also causes the resin to crystallize out of solution in a relatively short period of time. Consequently, conventional polyester resins prepared for use in coil coating compositions have been a compromise at best, with notable deficiencies.
Accordingly, there remains a need for an improved polyester resin which overcomes the foregoing deficiencies. Thus, it should impart to the finished cured coil coating film improved hardness and damage resistance but also improved bend flexibility, without a tendency to crystallize from solution. The resin should be easily soluble in common organic solvents.